The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of reducing phase aliasing (radial ripple) artifacts in magnetic resonance (MR) imaging. The present invention is also directed to a process of filling a partially filled blade of k-space.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Fast Spin Echo (FSE) imaging is an MR imaging technique commonly used as an efficient method of collecting MRI data with minimal artifact. But even FSE images suffer from ghosting artifacts resulting from voluntary or involuntary patient motion as the image acquisition usually takes a few minutes.
A number of imaging techniques has been developed to reduce motion artifacts of FSE images. One such FSE technique, which is referred to as Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging, encodes an MR signal by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution.
Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction has been shown to be very effective in reducing artifacts associated with inadvertent subject translational and rotational movements in near axial head scans. PROPELLER is relatively insensitive to motion as the center of k-space is sampled multiple times during acquisition. In addition, explicit correction for rotation and shift are used to further reduce motion artifacts. Notwithstanding the advantages of PROPELLER imaging, its applicability has been limited to axial head scans primarily as a result of radial ripple artifacts that have been observed in sagittal, coronal, and oblique (orientation significantly varying from axial) scans. These artifacts have also been observed in PROPELLER imaging of the liver and spine. To expand the applicability of PROPELLER to other slice orientations and other anatomies, these radial ripple artifacts must be reduced.
It would therefore be desirable to have a system and method of MR imaging implementing a PROPELLER or similar imaging protocol with a reduction of radial ripple artifacts.